1. Field of the Invention
This invention relates to improvements in a dental radiographic apparatus for photographing the entire jaw.
2. Prior Art
The present inventor previously filed a patent application in the form of U.S. patent application Ser. No. 667,769 on Nov. 2, 1984 for a radiographic apparatus for photographing the entire jaws capable of irradiating X-rays substantially normally to any position of the dental arch in proportion to the individual differences in the dental arch of a patient and which is free from ghost images in the mandibular ramuses. This previous invention, in which a locus of rotation of X-ray beam describes a substantial V-shaped envelope receding from the apex formed on the approximate medial line to the left and right of the medial line, changes the straight-line distance on the approximate medial line between the apex and receding point of limit of the envelope and structurally comprises fixed rails secured to a fixed frame and an arm freely movable on the rails. The invention was designed to change the straight-line distance by moving the arm with respect to the fixed frame. When this straight-line distance changes, a sectional plane curve also follows the change, with the result that there is a case wherein a patient must be positioned in the thus changed position. The reason for this is explained with reference to FIG. 1 of the accompanying l drawings of this invention. In the figure, the character designates an approximate medial line, and a designates an approximate dental arch of a patient, namely a tomographic plane curve (curved plane), and b designates an envelope described by a locus of movement of center of rotation of X-ray beam, the envelope being approximately V-shaped by being curved in a manner to recede left and right from the apex c of the appropriate medial line b. (In the previous invention, the envelope is shown as being approximately triangular, instead of the approximate V-shape, but this is merely a difference in the manner of expression and both expressions are substantially the same in content.) Now, when the arm is moved from this state by a mechanism to be later described, the apex c of the envelope b is moved to c.sub.1 and in proportion thereto, the envelope b is moved to b.sub.1. Thereupon, an effective radius r.sub.1 (a distance between an optional tangential line on the envelope and a curve a) extending from each position of the envelope b to the tomographic plane curve a also changes to r.sub.1. Consequently, a locus a.sub.1 of this effective radius r.sub.1 provides a new tomographic plane curve. Namely, the initial tomographic plane curve a must be shifted to position a.sub.1. For this reason, for example, according to a photographing method in which the front tooth region of a patient is brought into agreement with position a.sub.10 on the tomographic plane curve a.sub.1, it is impossible to make exact tomogram if this position a.sub.o is not shifted to the position a.sub.10 on the tomographic plane curve a.sub.1. In other words, when the apex c of the envelope is shifted to position c.sub.1 along the approximate medial line l, the initial tomographic curve a must also be shifted to position a.sub.1 in amounts corresponding to the straight-line distance of shift d thus made by shift of the apex c.
By the way, an attempt to change the envelope b in the manner described above was made for the first time by the present invention, and there has so far existed nothing or no means of indicating the aforestated amount of shift d in terms of a device.